The present invention generally relates to communication devices, and more particularly to a method and apparatus for a receiver to acquire a digital signal.
The present invention relates generally to frequency control systems and, more particularly, to a frequency control system operative in a digital receiver. The discontinuous transmission of signals can be due several events including start-up of the radiotelephone, dropping and re-establishing a call, and saving power during an idle state, among others.
Conventional frequency control circuitry controls frequency drift of the receiver by comparing zero-crossings of an intermediate frequency (IF) with an appropriately scaled version of a radio""s reference oscillator. The simplest form of this type of circuitry requires a signal to be applied continuously to the frequency control circuitry for proper operation thereof. Accordingly, frequency drift can occur in a receiver that is only intermittently powered to receive a signal. Moreover, output transitions from this circuit become very slow as the frequency error approaches zero. Therefore, the performance of this type of circuitry is clearly unacceptable if fast automatic frequency control (AFC) convergence is necessary.
Receivers constructed to receive digital signals oftentimes include decision-directed detectors. M-ary PSK modulation decision-directed detectors accumulate phase shift per symbol information. Such decision-directed devices are capable of providing a frequency control signal having a fast response. However, such frequency control signals generated by the decision-directed devices are accurate only when the frequency error of the reference oscillator viewed as accumulated phase error is small with respect to the phase decision space of the particular modulation. If the frequency error is large enough to cause the device to make an incorrect decision on the symbol received, then the device will direct the AFC algorithm to a false solution point. For example, the previously described conventional frequency control circuitry is operable when the frequency errors are within the reception bandwidth of the receiver, while conventional decision-directed devices are operable over frequency errors that are a fraction of the reception bandwidth. An example of this limitation is the QPSK modulation signal used in PDC which limits the useful range to only approximately xc2x12.6 kilohertz. This relationship depends upon the M-ary PSK nature of the modulation. A BPSK (2-ary PSK) signal would have been xc2x15.2 kHz while a 8-PSK signal would be xc2x11.3 kHz. BTW QAM signals are also within the capabilities of this technique.
The descriptions above show the restrictions and limitations on existing frequency control circuits and detectors. The associated complications make the AFC algorithm cumbersome and consequently, difficult to make robust. The lack of robustness has been demonstrated by forcing xe2x80x9cbadxe2x80x9d signal conditions and observing that AFC does not always recover when xe2x80x9cgoodxe2x80x9d signal conditions are restored.
Other problems occur when the received signal does not have uniform spectral density across the IF bandwidth. By its nature, the frequency detector steers the AFC algorithm such that the signal is xe2x80x9ccenteredxe2x80x9d in the IF passband. In this context, xe2x80x9ccenteredxe2x80x9d refers to equal signal power above and below the center of the IF passband. Consequently, problems may occur when received signals do not have uniform spectral density.
Further, all AFC algorithms can have a limited range of convergence that may not reliably lock on to a xcfx80/4 DQPSK or 8-PSK signal (such as is proposed in the next generation EIA/TIA standard 136A), which becomes further exacerbated at higher operating frequencies such as in the 1900 MHz frequency band. Because of this limited range of operability, currently known frequency control circuitry utilizing the decision-directed device cannot be utilized when the frequency differences between the signal transmitted to the receiver and the oscillation frequency of a receiver-oscillator is significant, such as immediately subsequent to initial powering of the receiver prior to effectuation of frequency control. Further, once frequency control is established the digital information in the mobile radiotelephone must be frame synchronized with its associated fixed-site base station.
Frame synchronization in today""s digital cellular telephones cannot take place until the frequency error of the reference oscillator is made sufficiently small to provide an unaliased unique (sync) word within data frames of incoming transmissions, which implies that an AFC algorithm must first reduce subscriber unit frequency error so that no symbol aliasing occurs. Because the AFC acquisition process is relatively slow compared to the data frame acquisition process, any useful information (after acquiring synchronization) may be held off for a significant length of time.
In a Time Division Multiple Access (TDMA) system, such as the Personal Digital Cellular (PDC) system in Japan, a subscriber unit must be capable of quickly synchronizing itself with the base station. Synchronization is required so that the subscriber unit can transmit and receive data during appropriate time slots. Frame synchronization is achieved by the successful reception of a unique word (also called xe2x80x9csync wordxe2x80x9d in the PDC system) for several consecutive frames. During the synchronization process, received data is correlated with a predefined unique word, and if correlation metric is greater than some threshold value, the assumption is made that the unique word has been received. Also, since the unique word within a frame occurs periodically, the subscriber unit has a time reference from which important TDMA and call processing events can be based.
However, in the presence of large frequency errors, the bit stream from a decision directed detector (including the unique word) can be xe2x80x98aliasedxe2x80x99. For example, in the well-known quadrature phase shift keying (QPSK) demodulation system, a received signal can be represented as an in-phase (I) signal and a quadrature signal (Q), which together represent a two-bit symbol having one of four states, 00, 01, 10 and 11. However, if the received signal accumulates sufficient phase shift due to frequency error, the symbol decision is incorrectly made.
FIG. 1 shows how symbol data, in the PDC system for instance, is aliased as a function of accumulated phase error per symbol. For example, if a true QPSK symbol of 00 accumulates about +xcfx80/2 radians of phase error (or within the phase space range of greater than +xcfx80/4 to less than +3xcfx80/4) during a symbol time, then it will be incorrectly decoded as the 01 symbol. Note that symbol aliasing is actually periodic with frequency error, i.e. QPSK symbol aliases repeat every integer multiple of 2xcfx80 radians phase error per symbol. It is not possible to distinguish between aliases caused by +xcfx80 and xe2x88x92xcfx80 radians phase error per symbol. Current receiver implementations can ignore larger errors, i.e. frequency errors which produce phase errors per symbol larger than xcfx80 radians, indicate some form of hardware failure and need not be addressed.
What is needed is a solution that shortens the time needed to acquire data frame synchronization. Such a solution will remove the following problems with the existing AFC/frame synchronization implementation: (1) slow response time of AFC convergence time; (2) incorrect response to accumulated phase error; and (3) slow signal processing start-up due to serial processing of AFC convergence followed by frame synchronization.
Accordingly, what is needed is a method and apparatus to shorten frame synchronization acquisition in a digital communication device when a signal is transmitted thereto in intermittent bursts even where frequency differences between such transmitted signal and the receiver-oscillator frequency is significant. It is also desirable to provide a solution that increases the AFC range of convergence without significantly adding to costs.